1. Field of the Invention
The present invention relates to wireless networks, and in particular to the decoding of the first high throughput (HT) symbol of the HT data field.
2. Related Art
The IEEE 802.11-2007 is a set of standards relating to wireless local area networks (WLAN). The legacy standards, e.g. 802.11a and 802.11g, have data rates that are relatively low. For example, both 802.11a (released 1999) and 802.11g (released 2003) have a data rate of 54 Mbit/s. In contrast, one of the most recent standards, 802.11n (projected release 2008), has a data rate of 300 Mbit/s. Thus, 802.11n is characterized as a high throughput protocol.
An 802.11 access point (AP) can operate in one of three modes: the legacy, mixed, or Greenfield mode. In the legacy mode, the AP can use one of the legacy protocols, e.g. 802.11a or 802.11g. In the mixed mode, the AP can use one of the legacy protocols or 802.11n. In the Greenfield mode, the AP can only use 802.11n (wherein “Greenfield” refers to a project that lacks any constraint imposed by prior work).
FIG. 1 illustrates an exemplary mixed mode packet format 100 that can be used in accordance with the 802.11n standard. As shown, mixed mode packet format 100 includes a legacy header 110, a high throughput (HT) header 111, and an HT data field 109. Legacy header 110 includes a legacy short training field 101, first and second legacy long training fields 102 and 101, and a legacy signal field 104. HT header 111 includes first and second HT signal fields 105 and 106, an HT short training field 107, and an HT long training field 108.
As known by those skilled in the art of wireless communication, fine timing (which determines an offset of the receiver) can be estimated from the slope of a frequency domain channel response. In a conventional legacy packet, which includes legacy header 110 followed by a legacy data field, legacy long training fields 102 and 103 can be used to consecutively better estimate the fine timing.
Notably, an HT receiver may ignore legacy header 110, thereby relying solely on HT header 111 for fine timing. However, HT header only includes one HT long training field 108. Unfortunately, by the time fine timing is obtained from this sole field, the first and second data symbols of HT data field 109 are already being processed for decoding. This fine timing is available for the third data symbol of HT data field 109.
To resolve this processing latency, the symbols of the data field could be stored in memory and then accessed for processing when the fine timing information is available. Unfortunately, this solution incurs undesirable additional hardware and time latency costs. Note that 802.11n requires that decoding of the packet must be complete by 16 microseconds after the end of the packet. Unfortunately, the additional time latency associated with data symbol storage and access could violate this requirement.
Therefore, a need arises for a technique to provide improved decoding for the first data symbol of the HT data field.